4 stroke and 2 stroke have nothing to do with the type of gas, or if there is a oil gas mix. It just means the amount of strokes in the engine per cycle. A 4 stroke can run on oil mix, and a 2 stroke can run on straight gas. Just depends on how they are designed.
Lots of old 2 stroke diesel engines out there that don't have oil added to the fuel.
Locomotives are a good example of 2 cycle diesel engines.
So far, most of the 4 stroke trimmers I have run across do have oil in the sump, It's only like 3 ounces, and they have a fill plug. This is true for Honda, Craftsman, Troy-Bilt, Ryobi, and huskys. The pics below show locations of oil fill/drain.
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On the other hand, I have also seen the ones that take mix, but have the full 4-stroke setup. (valves and camshaft). runs like a 4 cycle, lubricated like a 2 cycle.